The present invention is directed at an improved distillation process. More specifically, the present invention is directed at a method for increasing the yield of certain distillate fractions in a distillation system having a sidestream stripping zone.
In the separation of feeds such as hydrocarbon feeds into distillate fractions and a bottoms fraction, frequently it is desirable to maximize the production of one fraction. In distillation processes frequently a sidestream distillation zone, such as a sidestream stripper, is added to the column to facilitate the separation of the lighter fractions from the heavier fractions. Typically, the stripped out materials along with steam return to the main tower at the same tray or at a tray immediately above the liquid draw-off. However, changes in the feed composition, reflux ratio and/or product slate from that originally contemplated during the design of the distillation system may result in the operation of the system being non-optimal. For example hydraulic bottlenecks in the system often decrease the efficiency of the distillation system.
U.S. Pat. No. 3,210,271 discloses a distillation system having a main distillation zone and a sidestream stripping zone. Separated condensate is returned from the sidestream stripping zone to the main distillation zone through one of a plurality of inlets immediately above or below the liquid draw-off for the sidestream stripping zone.
U.S. Pat. No. 3,310,487 discloses a fractionation system employing a plurality of sidestream stripping zones in combination with a main distillation zone to process crude oil. The vapor fractions from the sidestream stripping zones are returned to the main distillation zone directly above the sidestream draw-off for that respective stripping zone.
Accordingly, it is desirable to provide a process to increase the flexibility of a distillation system having a sidestream stripping system.
It also would be desirable to provide a process which could improve the efficiency of the distillation system and thereby increase the production of desired distillate fractions.
The present invention is directed at improving the efficiency of certain distillation systems to thereby increase the production of select distillate fractions by returning the separated fraction from one sidestream separation zone to the main distillation zone at a location at least two trays and/or at least one theoretical stage above the tray from which the sidestream was withdrawn from the main distillation zone. In a preferred embodiment, at least a fraction of the material separated in the sidestream separation zone is returned to the main distillation zone at a location above that at which a lighter sidestream is withdrawn from the main distillation zone.